The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nepheline syenites which were intruded by a xenolithic syenite and a central plug of calcite to calcite-siderite carbonatite.Aegirine-augite, alkali feldspar and nepheline are the major mineral phases in the syenites, along with rare calcite.Temperatures of 680-910 C and silica activities of 0Á28-0Á43 were determined for the crystallization of the syenites on the basis of mineral equilibria.Oxygen fugacities, estimated using titanomagnetite compositions, were between 2 and 5 log units above the fayalite-magnetite-quartz buffer during the magmatic stage.Chondrite-normalized REE patterns of magmatic calcite in both carbonatites and syenites are characterized by REE enrichment (La CN -Yb CN ¼ 10-70).Calcite from the carbonatites has higher Ba ( $5490 ppm) and lower HREE concentrations than calcite from the syenites (54-106 ppm Ba).This is consistent with the behavior of these elements during separation of immiscible silicate-carbonate liquid pairs.e Nd (T ¼ 1Á30 Ga) values of clinopyroxenes from the syenites vary between þ1Á8 and þ2Á8, and e Nd ( T) values of whole-rock carbonatites range from þ2Á4 to þ2Á8.Calcite from the carbonatites has d 18 O values of 7Á8 to 8Á6 % and d 13 C values of À3Á9 to -4Á6 %. d 18 O values of clinopyroxene separates from the nepheline syenites range between 4Á2 and 4Á9%.The average oxygen isotopic composition of the nepheline syenitic melt was calculated based on known rock-water and mineral-water isotope fractionation to be 5Á7 Æ 0Á4%.Nd and C-O isotope compositions are typical for mantle-derived rocks and do not indicate significant crustal assimilation for either syenite or carbonatite magmas.The difference in d 18 O between calculated syenitic melts and carbonatites, and the overlap in e Nd values between carbonatites and syenites, are consistent with derivation of the carbonatites from the syenites via liquid immiscibility.